Polyvinylidenefluoride resin filaments are excellent from the point of view of tensile strength, impact resistance, weather resistance, and water resistance. They are used for various purposes, but mainly for fisheries supplies.
Conventionally, as filaments (fiber) used for fisheries supplies including fishing lines and fishing nets, synthetic resin materials were generally used for their economic efficiency, formability, excellent strength, or transparency.
Among them, polyamide resin (nylon) was mainly used in previous times. However, while their tensile strength or knot strength were excellent, these strengths decrease in water, and as their submergence (sedimentation property in water) is low and they turn yellow with time, polyolefin resin including polypropylene resin or polyvinylidenefluoride resin are now widely used instead of polyamide resin.
In particular, polyvinylidenefluoride resin filaments are excellent from the view of strength, impact resistance, and weather resistance or water resistance, and they have good submergence due to their high-density. Further, as their refractive index is similar to that of water and their surface reflection is low and transparency is high in water, they are highly appreciated as fisheries supplies.
Polyvinylidenefluoride resin filaments which are highly appreciated for fishing lines are highly demanded, further amelioration of performance or improvement of properties were consistently awaited. Thus, researches for ameliorating tensile strength, tensile elongation, and flexibility or knot strength, or what is called twist (or curl) of line (twisting (or curling) tendency) were made from previous times. As methods for ameliorating these problems, various propositions for improvement have been made and disclosed in patent publications and the like, wherein making polyvinylidenefluoride resin a composition with other resins, or a copolymer with other monomers, or combining with heterogeneous resins (making multilayer filaments by co-extrusion), or further performing a particular heat treatment or stretch treatment to filaments (see for example, patent documents 1 to 4).
Meanwhile, as mentioned in the above, while various ameliorations are made for ameliorating performance or properties of polyvinylidenefluoride resin filaments used for fisheries supplies including fishing lines, their use is limited to surf fishing or capturing small fishes. On the other hand, for use as fisheries supplies such as fishing lines, reflecting the recent tendency of consumers' diet, capture of large fishes including tuna or marlin are also important and highly demanded. Weight of these large fishes usually exceeds 100 kg, and the impact after fishes have taken the bait are rapid and enormous. These fishes are usually captured by longline fishing with a main line and a bait attached-branch line. However, for big game fishing using filaments of fishing line, significant tensile strength at break and impact resistance are required. Even by enlarging only the filament diameter in order to enhance the tensile strength at break, the tensile elongation at break that moderate impact resistance does not become appropriate. In filaments with a large diameter, cavities are often generated when formed. Thus, only few experiments to ameliorate filaments for fishing lines are made heretofore, and almost none is disclosed in patent publications.
In patent document No. 5, it is disclosed that a copolymer resin fiber of vinylidenefluoride and hexafluoropropylene, wherein a crystal structure with a particular parameter is determined is used as polyvinylidenefluoride resin filament with a large diameter of 0.5 mm or more, in order to avoid monofilament breakage against the rapid impact force when catching heavy weight fishes.
From this technical situation, universal fishing methods for capturing efficiently and surely large fishes including tuna or marlin, which are highly consumed recently, are becoming important. Responding to this, in order to assure capture of large fishes with fishing line made by polyvinylidenefluoride resin, development of polyvinylidenefluoride resin filament as fishing line for capturing large fishes, wherein even by enhancing significantly the tensile strength at break by enlarging the filament diameter than that of usual fishing line, up to a large diameter of 0.5 to 3 mm, an appropriate tensile elongation at break can be obtained, so that the rapid and enormous impact generated when large fishes have taken the bait can be reduced, and a homogenous fiber structure can be obtained which generates almost no inner cavity when forming a filament with a large diameter, is awaited.
[Patent document 1] Japanese Patent Publication No. 4-61087 (Claims; page 1, left column, lines 18-19)
[Patent document 2] WO02/064867 (page 1, lines 21-26; page 22, lines 3-5)
[Patent document 3] Japanese Laid-Open Patent Application No. 7-292519 (Abstract)
[Patent document 4] Japanese Laid-Open Patent Application No. 5-148707 (Abstract)
[Patent document 5] Japanese Laid-Open Patent Application No. 7-216635 (Abstract; claims; paragraphs 0003-0005)